The present invention relates to a mirror, and more particularly, to a multi-layered surface mirror having superior properties in terms of such aspects as freedom from glare, visibility and decorative finish.
The multi-layered surface mirror of the present invention is useful in such applications as automotive rearview mirrors, mirrors with a convex surface used for warning drivers against hazards in the road ahead, and ornamental mirrors.
Conventional mirrors such as automotive rearview mirrors are generally composed of a glass substrate coated on its back side with a coating of a metal such as aluminum or chromium, which is further overlaid with a protective coating.
Multi-layered back reflecting mirrors have recently been marketed. Such mirrors are composed of a glass substrate having a dielectric multi-layered coating on its back side and which is overlaid with a black light-absorbing coating. Multi-layered surface mirrors have also been proposed to be composed of a glass substrate having a dielectric multi-layered coating on its front side and a black light-absorbing coating on the back side. In the mirror of the former type, an example of which is disclosed in Unexamined Published Japanese Patent Application No. 144504/1982, the dielectric multi-layered coating in composed of five alternating films of TiO.sub.2 and SiO.sub.2 that have optical thickness of .lambda./4 or 3.lambda./4, or a combination of such layer. Another example of a multi-layered back reflecting mirror is disclosed in Unexamined Published Japanese Patent Application No. 98405/1985 explaying dielectric multi-layered coating having four alternating films of TiO.sub.2 and SiO.sub.2, each having an optical thickness of n.lambda./4 (where n is an odd number). An example of a surface reflecting mirror of the second type employs a three-layered dielectric coating of TiO.sub.2 --SiO.sub.2 --TiO.sub.2, each layer having an optical thickness of .lambda./4. Commercial products of this type of mirror are believed to have been manufactured in West Germany.
The above-described prior art mirrors have spectral reflection characteristics as shown in FIG. 4, in which curve (a) refers to an aluminum mirror. As curve (a) shows, the aluminum mirror has a high reflectance (80 to 90%) and flat reflection characteristics which are not dependent upon wavelength. If this aluminum mirror is used as a rearview mirror in an automobile, it causes strong reflection of the light flux from the headlights of a following vehicle, which leads to eye fatigue because of the glare caused by the reflected light.
The spectral reflection characteristics of the prior art chromium mirror are shown by curve (b) in FIG. 4. As curve (b) shows, the chromium mirror has a low reflectance (38-50%) and exhibits a certain degree of freedom from glare compared with the higher reflectance aluminum mirror. However, the glareless property of the chromium mirror is not as high as it might be expected to be because of the flat spectral characteristics of the mirror. In addition, the visibility of the chromium mirror is rather poor.
The dielectric multi-layered back reflecting mirrors disclosed in Unexamined Published Japanese Patent Application Nos. 144504/1982 and 98405/1985 have spectral reflection characteristics as shown by curves (c) and (d), respectively, in FIG. 4. These spectral reflection characteristics are similar to those of a commercial multi-layered surface reflecting mirror shown by curve (e) in FIG. 4. In general, the visibility of these mirrors and their freedom from glare are not satisfactory for the same reasons as noted below for the commercial multi-layered surface reflecting mirror. Furthermore, these mirrors which are of the back reflecting type, produce reflection from the front surface of glass and the resulting image overlap impairs resolution.
A known commercial multi-layered surface reflecting mirror is composed of a glass substrate that has a three-layered (TiO.sub.2 --SiO.sub.2 --TiO.sub.2) coating former on the front side and a light-absorbing coating on the back side. The optical thickness of each layer of the coating is .lambda./4. This three-layered mirror has spectral reflection characteristics as shown by curve (e) in FIG. 4. The mirror has a reflectance of 48% and satisfies the reflectance requirement specified in JIS (Japanese Industrial Standard) D 5705 "Automotive Mirror System" for a minimum reflectance of 38% for the chromium mirror. However, as is clear from curve (e) in FIGS. 4, the reflectance of this mirror peaks in the range of 430-550 nm and drops sharply in the longer ranges up to 700 nm, causing the mirror to reflect bluish light. As a result, the color balance provided by the mirror is disturbed in such a way that reds are particularly difficult to recognized. Thus, the visibility of the mirror is impaired.
The spectral luminous efficiency V'(.lambda.) of the human eye, the spectral energy characteristics P(.lambda.) of a headlight of an automobile, and the product of P(.lambda.).times.V'(.lambda.) are shown by three different curves in FIG. 5. As these curves show, the response of a driver's eyes to the headlights of a closely following vehicle at night is highest at a wavelength between 480 and 550 nm. Since this range coincides with the region where the peak of the spectral reflectance curve (e) in FIG. 4 occurs, the ability of the commercial three-layered mirror and other conventional surface reflecting mirrors to prevent glare from the headlights of a following vehicle at night is not as high as desired.